(1) Field of the Invention
The present invention relates to a so-called "tandem-type" image forming apparatus where a plurality of image holding components, such as photosensitive drums, are set along the transportation path of a recording sheet.
(2) Related Art
Tandem-type image forming apparatuses have received much attention in recent years because of their ability to perform color printing at high speed.
A color copying machine, as one example of this type of image forming apparatus, has four photosensitive drums corresponding to four colors set along the transportation path of a recording sheet and a transfer unit. The transfer unit is set under the photosensitive drums and includes a transport belt that runs over a plurality of rollers and transfer chargers respectively facing the photosensitive drums. Toner images of cyan, magenta, yellow, and black separately formed on the photosensitive drums are sequentially transferred by the transfer chargers onto a recording sheet transported by the transport belt. As a result, four color images are superimposed on the recording sheet to form a color image. In general, the photosensitive drum used for forming a black image is set at a rearmost position on the transportation path of the recording sheet for better reproduction of black parts of the color image.
When performing operations aside from full-color image formation, such as when forming a black image formation using this type of image forming apparatus, toner images are not formed on the photosensitive drums for cyan, magenta, and yellow, and a toner image is formed only on the photosensitive drum used for the black image formation.
However, when only one photosensitive drum is used, the recording sheet still comes into contact with the other three photosensitive drums during transportation. For this reason, the three photosensitive drums which are not used for the image formation still need to be rotated. This results in unnecessary wear and tear on the photosensitive drums and cleaning blades that are in contact with the photosensitive drums. Against this backdrop, Japanese Laid-Open Patent Application No. 3-288173 teaches an example of a color image forming apparatus which tilts the whole transfer unit from the horizontal position when black image formation is performed, so that the transport belt does not come into contact with the photosensitive drums for C, M, and Y which are not used for black image formation.
Accordingly, the photosensitive drums of cyan, magenta, and yellow do not need to be rotated when a black image is formed. This prevents unnecessary wear and tear on these photosensitive drums.
Japanese Laid-Open Patent Application No. 3-288173 also teaches that a backup plate may be provided in the transfer unit for each photosensitive drum to improve the transfer of the toner images formed on the photosensitive drums. Each backup plate is an elastic member which presses the transport belt against the photosensitive drum immediately before a transfer position, so that the recording sheet transported by the transport belt is tightly pressed against the surface of the photosensitive drum before the recording sheet enters a transfer area. As a result, each toner image is reliably transferred onto the recording sheet.
With this conventional image forming apparatus, however, the transport belt is separated from the photosensitive drums by the shift of the whole transfer unit which is provided with the transfer chargers. This causes variations in the positions of the transfer chargers relative to the photosensitive drums during the image formation. As a result, image transfer is unstable.
Moreover, with this conventional image forming apparatus, the backup plates of the photosensitive drums of cyan, magenta, and yellow still push up the transport belt when forming a black image. This prevents the transport belt from running smoothly, and accordingly, a satisfactory transferred image cannot be obtained.
In addition, the position at which the photosensitive drum used for black image formation comes into contact with the transport belt, that is, the transfer position, is different when forming a black image than when forming a full-color image. As a result, image transfer is unstable.